Initial stages and progression of arteriolar changes which may be associated with the pathogenesis of hypertensive vascular disease, will be examined during the development of elevated arterial pressure in SHR rats by a combination of intravital and electron microscopy. To study whether or not the hypertension associated changes appear simultaneously, are the same, and show similar progression in different arteries, samples of retina, kidney, diaphragm, skin, and mesentery will be investigated in successive developmental stages of hypertension. Results from the electron microscopical and intravital microscopical studies will be subjected to stereological and statistical analysis and compared with results from similar studies using normotensive control animals. Knowledge of chronology of the vascular changes will help to identify which of the changes are primary and therefore responsible for the elevated pressure, and which are secondary resulting from the hypertension.